The term “Optical Coherence Tomography”, OCT, defines a group of optical measuring techniques wherein the (limited) coherence length of light is used for high precision distance measurements on reflective surfaces. Especially for medical applications, such as ophthalmologic diagnosis and monitoring, OCT has proved a valuable tool, since it allows precise and non-invasive measurements down to several millimeters under the surface of the body. Moreover, a high longitudinal precision of a measurement, on a few-micrometer scale, can be achieved even at a relatively long distance between the OCT apparatus and the measured object.
A single OCT measurement usually provides information only about average characteristics of the reflecting area. However, a topography of a reflecting surface can be resolved in great detail, if for a single measurement the size of the measured area is reduced as far as possible and if a multitude of such point-like measurements are performed densely distributed over the surface. In established OCT techniques, an extended surface measurement is thus typically performed in the manner of an x-y-scan, also named “B-scan”, on a grid of equally spaced measuring points. For the single point measurements, however, various techniques of optical coherence tomography have been developed to optically determine the distance between the measured point and a reference point, typically inside the OCT device.
Conventional methods of OCT may involve an interferometer setup, wherein a generated light beam becomes divided into a sample beam and a reference beam. While the propagation of the reference beam is confined to an interior of the apparatus, the sample beam is emitted from the device towards the measured sample and, after reflection by the sample, re-enters the interferometer. There, the reflected sample beam and the reference beam are superimposed and, if a difference in the path length of the two beams lies below a coherence length of the used light, the superposition of the beams will produce detectable interference. After detection by means such as a photo-diode or a spectrograph, the interference can be analyzed, for example, with regard to a difference in the spectral intensity or a difference in the path length of the two beams. As a result of that analysis various sample characteristics, such as a reflectivity and a distance, of the reflecting surface can be determined. Aside from single surfaces, the described techniques also allow for simultaneous characterization of a plurality of stacked and partially reflective planes in the sample.
Despite the aforementioned common features, practical OCT apparatus may differ from one another, for example, in the details of their setup, in the use of a wide or a narrow bandwidth light source, in the detected or the analyzed signal characteristics, in the employed analyzing algorithm, etc. According to a conventional classification scheme, OCT techniques may be distinguished by their setup, into “sequential” techniques, if a measurement includes a plurality of detection processes with a controlled variation of the optical path length of the reference arm (“scanning-arm”) or of the used wavelength (“swept source”), and into “simultaneous” techniques, if a measurement may be performed by a single detection process only, in which cases usually a spectrogram of the superimposed reference and sample beams is recorded. Alternatively, OCT techniques may be distinguished by the prevailing method of data acquisition and processing, into what is called “time domain OCT”, TD-OCT, which is usually performed in connection with an interferometer of the scanning-arm type, and into “frequency domain OCT”, FD-OCT, which includes a processing of spectrally resolved interference information.
Recently, a variant of frequency domain, FD-, OCT has been suggested, which became known as “dispersion encoded full range optical coherence tomography”, DEFR-OCT. In DEFR-OCT, a well-defined difference in dispersion between the two interferometer arms is purposefully introduced. In connection with particular processing algorithms, that dispersion imbalance has been shown to allow for a more efficient and convenient retrieval of the depth information from a Fourier-transformed spectrogram.